1. Field of the Invention
The present invention relates to fine metal particles which have excellent electrical conductivity because iron is contained therein in a specific amount and which have excellent dispersion stability and are less apt to suffer ionization, particle growth, or the like in conductive coating films, and to a coating liquid for forming transparent conductive coating films which contains the fine metal particles and has a long pot life. The invention further relates to a substrate with transparent conductive coating film which is obtained with the coating liquid for forming transparent conductive coating films and is excellent in antistatic properties, electromagnetic shielding properties, reliability, and durability. The invention furthermore relates to a display device having a front panel comprising the substrate with transparent conductive coating film.
2. Description of Related Art
A transparent coating film having the functions of preventing static buildup and preventing reflection has hitherto been formed on a surface of the transparent substrate of a display panel for cathode ray tubes, fluorescent character display tubes, or liquid-crystal displays for the purposes of preventing static buildup and reflection on the surface.
Incidentally, influences of electromagnetic waves emitted from cathode ray tubes and the like on the human body have recently become a problem. Besides the prevention of static buildup and reflection for which measures have been taken, it is desired to shield from those electromagnetic waves and to diminish the electromagnetic field produced by the emission of electromagnetic waves.
One method for shielding from those electromagnetic waves or the like is to form a conductive coating film for electromagnetic wave shielding on a surface of the display panel of a cathode ray tube or the like. However, the conductive coating film for electromagnetic shielding has been required to have a surface resistance as low as 102 to 104 Ω/□, in contrast to the existing antistatic conductive coating films for which a surface resistance of about 107 Ω/□ or lower suffices.
When a coating liquid containing a conductive oxide heretofore in use, such as Sb-doped tin oxide or Sn-doped indium oxide, is used for forming a conductive coating film having such a low surface resistance, it is necessary that the film is formed in a larger thickness than in the case of existing antistatic coating films. However, since a conductive coating film produces an anti-reflection effect only when it has a thickness of about from 10 to 200 nm, use of an existing conductive oxide such as Sb-doped tin oxide or Sn-doped indium oxide results in a coating film having heightened surface resistance. There has hence been a problem that it is difficult to obtain a conductive coating film having not only excellent electromagnetic wave shielding properties but excellent anti-reflection properties.
Another method for forming a conductive coating film having low surface resistance is to use a conductive-film-forming coating liquid containing fine particles of a metal such as Ag to form on a surface of a substrate a coating film containing the fine metal particles. In this method, a dispersion of colloidal fine metal particles in a polar solvent is used as the film-forming coating liquid containing fine metal particles. In such coating liquids, the surface of the fine metal particles has been treated with an organic stabilizer such as poly(vinyl alcohol), polyvinylpyrrolidone, or gelatin in order to improve the dispersibility and stability of the colloidal fine metal particles. However, the conductive coating film formed from such a film-forming coating liquid containing fine metal particles has a drawback that since the fine metal particles in the coating film are in contact with one another through the organic stabilizer, the interparticulate resistance is high and, hence, the surface resistance of the coating film cannot be low. It is therefore necessary to conduct burning at a temperature as high as about 400° C. after the film formation to decompose and remove the stabilizer. However, the burning at high temperatures for decomposition and removal of the stabilizer encounters a problem that fusion and aggregation of fine metal particles occur to thereby reduce the transparency of the conductive coating film and heighten the haze thereof. Furthermore, in the case of cathode ray tubes and the like, there also has been a problem that exposure to high temperatures cause deterioration.
The existing transparent conductive coating film containing fine particles of a metal such as Ag has further had a problem that metal oxidation and particle growth due to ionization may occur and, in some cases, corrosion occurs, whereby the coating film is reduced in conductivity or light transmittance to impair the reliability of the display device.
The applicant proposed in, e.g., JP-A-10-188681 a coating liquid for forming transparent conductive coating films which contains fine composite metal particles comprising two or more metals and having an average particle diameter of from 1 to 200 nm. However, with such fine composite metal particles, it has been difficult to obtain a coating liquid having a sufficiently long pot life.
The present inventors made further investigations on fine metal particles. As a result, they have found that when a specific amount of Fe is incorporated into fine metal particles, the coating liquid for forming transparent conductive coating films has enhanced stability and gives a transparent conductive coating film having excellent durability. The invention has been thus completed.
In JP-A-11-80619,there is a description to the effect that when a slight amount of Fe is contained as an impurity, then the transparent conductive coating film formed has a more even distribution of conductivity in the surface and has lower resistance. JP-A-11-80619 further contains a description to the effect that the content of Fe in the fine metal particles is in the range of from 0.0020 to 0.015% by weight. There also is a description to the effect that film-forming properties become poor as the content of Fe increases. However, the fine metal particles described in JP-A-11-80619 have had a problem that the coating liquid has insufficient stability and gives a film having insufficient strength and poor durability.
An object of the invention is to overcome the problems of the related art described above and to provide fine metal particles which have a surface resistance as low as about 102 to 104 Ω/□, are excellent in antistatic properties, anti-reflection properties, and electromagnetic shielding properties, and are suitable for use in forming a transparent conductive coating film excellent in reliability and durability. Another object is to provide a process for producing the fine metal particles, a coating liquid for forming transparent conductive coating films which contains the fine metal particles, a substrate with transparent conductive coating film, and a display device having the coated substrate.